A collection of studies to be published in the November 4 issue of the Journal of the American Medical Association offer insights into the H1N1 flu strain that has now caused the deaths of at least 4,500 people worldwide, and which World Health Organization officials estimate will continue to be classified as a pandemic for several years. A breakdown of the major findings:

Critical illness caused by H1N1 sets in quickly: A study of 128 Canadian patients with confirmed or probable cases of swine flu found that, critical illness—including organ failure, plummeting levels of oxygen in the blood and the need for mechanical respiratory assistance—tends to set in shortly after initial hospitalization. Most patients included in the study experienced symptoms of the H1N1 flu virus for about four days before going to the hospital, but upon admission generally deteriorated into critical condition within one to two days. In this study, 14.3% of patients (24 people) died within a month after critical illness set in; five more patients (fewer than 3%) died within three months after the onset of critical illness.

Younger patients and women may be hardest hit by H1N1: In the Canadian study, the average age of patients with confirmed or likely cases of the H1N1 flu virus was 32.3-years-old, and 67.3% (113 patients) were women. Children too made up a large portion of swine flu patients—29.8% (50 patients). In contrast, few people older than 60 were admitted to the hospital for swine flu during the study period. The concentration of illness among younger patients is strikingly similar to the 1918 pandemic of the same flu strain, the researchers say. “[S]evere disease and mortality in the current outbreak is concentrated in relatively healthy adolescents and adults between the ages of 10 and 60 years, a pattern reminiscent of the W-shaped curve previously seen only during the 1918 H1N1 Spanish pandemic,” they write.

Death toll highest among patients who become critically ill: A study of 899 patients admitted into hospitals in Mexico with confirmed or suspected cases of swine flu revealed that those in whom the virus caused critical illness faced the most grim outcomes. Of the 58 patients who became critically ill (just 6% of all admitted), 41.4% died within three months. Of those, 19 died within two weeks of admission. Again, the researchers found, the critically ill population consisted of mostly younger patients—the median age for those in serious condition was 44 years.

Oxygenation treatment is largely successful for H1N1 patients with respiratory complications: Finally, good news! A study of 68 patients with severe respiratory symptoms of H1N1 admitted to 15 different intensive care units across Australia and New Zealand between June 1–August 31 of this year, found that, when treated with extracorporeal membrane oxygenation (ECMO), the majority of patients survived. ECMO—a technique that enables doctors to oxygenate patients’ blood outside of the body—was used to treat patients for 7-10 days. By the end of the study period, 71% of patients (48 people) had survived, with 32 already discharged from the hospital. “Despite the disease severity and the intensity of treatment, the mortality rate was low,” the researchers write. At the end of reporting, 14 patients (21%) had died, and 6 remained in intensive care.